Liquid ink printers of the type frequently referred to as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal, have at least one printhead from having drop ejectors which droplets of ink are directed towards a recording sheet. Within the printhead, the ink is contained in a plurality of channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
In a thermal ink-jet printer, the power pulses are usually produced by resistors, each located in a respective one of the channels, which are individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially the ink bulges from the channel orifice. The bubble quickly collapses and the ink within the channel then retracts and separates from the bulging ink thereby forming a droplet moving in a direction away from the channel orifice and towards the recording medium whereupon hitting the recording medium a dot or spot of ink is deposited. The channel is then refilled by capillary action, which, in turn, draws ink from a supply container of liquid ink. Operation of a thermal ink-jet printer is described in, for example, U.S. Pat. No. 4,849,774.
The ink jet printhead may be incorporated into either a carriage type printer, a partial width array type printer, or a page-width type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is attached to a carriage which is reciprocated to print one swath of information (equal to the length of a column of nozzles), at a time, on a stationary recording medium, such as paper or a transparency. After the swath is printed, the paper can be stepped a distance equal to the height of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith. This procedure is repeated until the entire page is printed. One such printer is the Xerox 4004 which includes a cammed scan carriage which is manually moved by a user to move the scan carriage with respect to a print platen to provide for printing of different thicknesses of recording medium.
In contrast, the page width printer includes a stationary printhead having a length sufficient to print across the width or length of a sheet of recording medium at a time. The recording medium is continually moved past the page width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process. A page width ink-jet printer is described, for instance, in U.S. Pat. No. 5,192,959.
Printers print information received from an image output device such as a personal computer. Oftentimes, this received information is in the form of a raster scan image such as a full page bitmap or in the form of an image written in a page description language. The raster scan image includes a series of the scan lines or rows consisting of bits representing pixel information in which each scan line or row contains information sufficient to print a single fine line of information across a page in a linear fashion. Printers can print bitmap information as received. If a printer receives an image written in the page description language, however, the printer or the image input device converts the page description language to a bitmap consisting of pixel information.
The density of information contained in the full page bitmap can correspond to the density of the image to be printed by the liquid ink printer. For instance, in a thermal ink jet printhead printing at 300 spots per inch, the full page bitmap will have information enabling the printhead to print at the required density. Known printers also manipulate image bitmaps to print at resolutions greater than or less than the resolution of the received image.
In reciprocating carriage printers, image defects can occur due to non-uniform absorption and drying of the ink. These image defects can be reduced by printing the image in more than one pass of the printhead, including where each pass prints a portion of the pixels in a dot pattern known as a "checkerboard" pattern. In this type of two pass printing, a first pass of the printhead carriage prints a swath of information in which odd numbered pixels of odd numbered rows or scanlines and even numbered pixels of even numbered rows or scanlines of a bitmap are printed. In a second pass of the carriage printhead, the complementary pattern consisting of even numbered pixels in odd numbered rows and odd numbered pixels in even numbered rows is printed. By printing in two passes, the ink printed in the first pass has time to dry partially before the ink from the second pattern is deposited.
Various methods and apparatus of printing with liquid ink printers are described in the following disclosures which may be relevant to certain aspects of the present invention.
In U.S. Pat. No. 4,748,453 to Lin et al. a method of depositing spots of liquid ink upon selected pixel centers is described. A line of information is printed in at least two passes so as to deposit spots of liquid ink on selected pixel centers in a checkerboard pattern wherein only diagonally adjacent pixel areas are deposited in the same pass.
U.S. Pat. No. 4,965,593 to Hickman describes a dot printer wherein the spacing of ink jet nozzles of a print head are spaced by an amount greater than the pixel spacing of the printing medium such that adjacent pixels are not printed until the deposited colorant has time to dry.
U.S. Pat. No. 4,967,203 to Doan et al. describes an interlace printing process for an ink jet printer. Printed images are produced by staggering applications of ink dots to pixel locations such that overlapping ink dots are printed on successive passes of a printhead and such that swaths are partially printed on overlapping passes of the printhead. Multi-colored or multi-shaded images are completed by grouping pixels into superpixels and applying various combinations of colored ink dots to the various pixels within each superpixel in a staggered sequence.
U.S. Pat. No. 4,999,646 to Trask describes a method for enhancing the uniformity and consistency of dot formation produced by color ink jet printing. A multiple pass complementary dot pattern ink jet printing process uses successive printed swaths made by depositing first and second partially overlapping complementary dot patterns on a print media.
Japanese Laid Open publication number 60-107975, laid open Jun. 13, 1985, describes an ink jet recording apparatus including array means where dots are arrayed in such a manner that every other column is printed with alternation of an odd row and an even row in a first scan and portions not printed by the first scan are printed by the second scan.